Manufacturing Bits: Jan. 13

Pop-up 3D printing
Northwestern University and the University of Illinois at Urbana-Champaign have developed a new “pop-up” printing technique to make 3D structures down to 100nm.

The technique has advantages over today’s 3D printing, which is creating a lot of buzz, if not hype, in the market. Researchers from Northwestern and Illinois devised a printing technique that mimics the action of a children’s pop-up book. The structure starts as a flat 2D structure and then pops up into a 3D structure.

Using silicon and other materials, researchers produced more than 40 different designs, including shapes resembling a peacock, flower, starburst, table, basket, tent and starfish.

Northwestern University and University of Illinois have developed a simple new fabrication technique that mimics the action of a children’s pop-up book. An experimental image of a flower-like structure is shown. (Source: University of Illinois)

The technique is fast and inexpensive. In contrast, 3D printing is difficult to integrate more than one material in a structure, according to researchers. “A key, unique feature of these approaches to 3D microarchitectures is that they work equally well with a very wide variety of materials, including the highest performance semiconductors, such as device-grade silicon, and fully formed, state-of-the-art planar devices and systems,” said John Rogers, a professor of materials science and engineering at the University of Illinois, on Northwestern’s Web site. “We believe, as a result, that these ideas have relevance to nearly every class of microsystem technology–from electronics to photonics, optoelectronics, microelectromechanical structures and others.”

Hot plate chips
The University of Würzburg has used a novel technique to make tiny organic semiconductors. Researchers used a hot plate to make molecules with a high electron mobility.

With the hot plate, researchers made n-channel single-crystal field-effect transistors and organic thin-film transistors. The transistors are based on a material called dichloro naphthalene diimide substituted with fluoroalkyl chains.

The organic n-semiconductor, a dichlorinated naphthalene diimide produced in the lab, can be sublimated under ambient conditions, namely in air. This, in turn, forms mono-crystals that have a new molecule arrangement.

The organic n-semiconductor, a dichlorinated naphthalene diimide produced in the lab, can be sublimated under ambient conditions, namely in air. (Graphic: Matthias Stolte)

According to researchers, the material holds the record among small molecules in terms of the charge-carrier mobility of electrons in air. Researchers were able to fabricate single-crystal FETs with electron mobilities in air of up to 8.6 cm2 V−1 s−1 (α-phase) and up to 3.5 cm2 V−1 s−1 (β-phase) on n-octadecyltriethoxysilane-modified substrates.

Researchers used a different method to make these structures. Typically, organic semiconductors are either vacuum-processed or they are printed from a liquid solution. But vacuum-based processing is expensive. And the solvent-based method impacts the quality of the layers.

In contrast, researchers used a hot plate, which causes the molecules to form a different pattern. Under these conditions, a new polymorph with two-dimensional brick-wall packing mode (β-phase) is obtained. This is different than the previously reported herringbone packing motif obtained from a solution (α-phase).

The process is simple. “This semiconductor can be produced and processed under ambient conditions. And what is more, it is stable when exposed to air,” said Matthias Stolte at the University of Würzburg, in a statement. “We place the material on a substrate onto a hot plate heated to 180 degrees Celsius. Then, if you position a second substrate next to the first, the semiconductor will deposit there in a mono-crystalline layer.”

Repulsive gel
Riken, the National Institute of Material Science and the University of Tokyo have developed a new hydrogel. The gel enables electrostatic repulsion, rather than attractive properties. The gel has the same force that makes our hair stand on end when subjected to static electricity.

Researchers discovered the gel when titanate nano-sheets were suspended in an aqueous colloidal dispersion. The nano-sheets align themselves face-to-face in a plane when subjected to a strong magnetic field. “The field maximizes the electrostatic repulsion between them and entices them into a quasi-crystalline structure,” according to researchers. “They naturally orient themselves face to face, separated by the electrostatic forces between them.”

In a statement, Yasuhiro Ishida, head of the Emergent Bioinspired Soft Matter Research Team at Riken, said: “This was a surprising discovery, but one that nature has already made use of. We anticipate that the concept of embedding anisotropic repulsive electrostatics within a composite material, based on inspiration from articular cartilage, will open new possibilities for developing soft materials with unusual functions. Materials of this kind could be used in the future in various areas from regenerative medicine to precise machine engineering, by allowing the creation of artificial cartilage, anti-vibration materials and other materials that require resistance to deformation in one plane.”